1. Field of the Invention
The present invention generally relates to devices and methods for measuring fluid density and other fluid flow properties in a flow stream, where fluid is taken to mean any liquid, gas, or mixture thereof, including those which contain solids. More particularly, the present invention relates to a high accuracy density and viscosity measurement device suitable for use in a high-temperature, high-pressure, high-shock environment such as may be encountered in a wellbore.
2. Description of the Related Art
There are many instances in industrial processes and controls for handling flowing fluids where the density of the moving fluid has to be determined accurately. One particular application is in the identification of reservoir fluids flowing in a well. Water often co-exists with crude oil in some common geologic formations. As such, both substances are often pumped up together by a working oil well and the water is ultimately separated from the crude oil at a downstream location. It is desirable to determine the amount of oil that occurs in an oil-water stream flowing from a formation. To accurately determine the amount of crude oil extracted from a formation, a xe2x80x9cnet oil computerxe2x80x9d may be used to ascertain the amount of crude oil. The xe2x80x9cnet oil computerxe2x80x9d determines the total volume flow rate of the flow stream and calculates the flow stream""s oil percentage (based on density measurements) to determine the net amount of oil that emanates from the formation. Given the large quantities of crude oil that are usually involved, any small inaccuracies in measuring density can disadvantageously accumulate over a relatively short interval of time to become a large error in a totalized volumetric measure.
Another particular application of density measurement is to determine the mass flow rate of a fluid medium. Mass flow rate can be calculated as a product of a fluid density (determined by a density meter) and a volume flow rate of the fluid (measured by a volumetric flowmeter). There are mass flowmeters available at the present time such as the Coriolis force or convective inertia force mass flowmeters and thermal probe mass flowmeters. These types of mass flowmeters, while they function excellently in the mass flow measurement of low viscosity fluids, work poorly in measuring flows of highly viscous fluids. The fluid""s viscosity introduces error in the data acquisition for the mass flow rate. One of the more promising approaches to measurement of the mass flow rate is to combine an accurate density meter and a reliable positive displacement volumetric flowmeter. This combination is particularly effective in measuring mass flow rates of highly viscous fluids or mixtures of fluids and gasses.
Coriolis mass flow meters can be used to measure the density of an unknown process fluid. In general, as taught, for example, in U.S. Pat. No. 4,491,025, a Coriolis meter can contain two parallel conduits, each typically being a U-shaped flow tube. Each flow tube is driven such that it oscillates about an axis. As the process fluid flows through each oscillating flow tube, movement of the fluid produces reactionary Coriolis forces that are perpendicularly oriented to the plane of the fluid""s angular velocity in the tube. These reactionary Coriolis forces cause each tube to twist about a torsional axis that, for U-shaped flow tubes, is normal to its bending axis. The net effect is a slight deformation and deflection of the conduit proportional to the mass flow rate of the fluid. This deformation is normally measured as a small difference between the deflection at the inlet ends of the conduits compared to the deflection at the outlet ends. Both tubes are oppositely driven such that each tube behaves as a separate tine of a tuning fork and thereby advantageously cancels any undesirable vibrations that might otherwise mask the Coriolis forces. The resonant frequency at which each flow tube oscillates depends upon its total mass, i.e. the mass of the empty tube itself plus the mass of the fluid flowing therethrough. Inasmuch as the total mass will vary as the density of the fluid flowing through the tube varies, the resonant frequency will likewise vary with any changes in density.
As specifically taught in U.S. Pat. No. 4,491,009, the density of an unknown fluid flowing through an oscillating flow tube is proportional to the square of the period at which the tube resonates. While the circuit taught in this patent may provide accurate density measurements, it unfortunately possesses several drawbacks. First, for certain applications, density measurements to an accuracy of one part in 10,000 are necessary. An accuracy of this magnitude is generally not available through an analog circuit unless highly precise analog components are used. Such components are quite expensive. Second, the analog circuit disclosed in this patent cannot be independently calibrated to compensate for changing characteristics of the electronic componentsxe2x80x94such as offset, drift, aging and the like. Specifically, this circuit is calibrated on a xe2x80x9clumpedxe2x80x9d basis, i.e. by first passing a known fluid, such as water, through the meter and then adjusting the circuit to provide the proper density reading at its output. This process compensates for any errors that occur at the time of calibration that are attributable either to physical errors in measuring density using a Coriolis mass flow meter or to errors generated by the changing characteristics of the electrical components themselves. Unfortunately, after the circuit has been calibrated in this fashion, component characteristics will subsequently change over time and thereby inject errors into the density readings produced by the circuit. This, in turn, will eventually necessitate an entire re-calibration.
All densitometers are generally calibrated using a calibration fluid having a known density. This density is specified at a certain temperature. Unfortunately, the density of most fluids varies with temperature; some fluids exhibit a significant variation, while other fluids exhibit relatively little variation. Consequently, many currently available densitometers require that the temperature of the calibration fluid must be carefully controlled before the fluid is injected into the densitometer for calibration. This necessitates that the container holding the fluid must be placed in a temperature bath for a sufficiently long period of time so that the fluid will stabilize to a desired temperature. In addition, provisions must be made to ensure that the temperature of the fluid will not change as the fluid is pumped through the meter. Accurately controlling the temperature of a fluid and then accurately maintaining its temperature, while the fluid is being pumped through the meter, is both a costly and tedious process.
It may be appreciated from the foregoing that a need exists in the art for a high-accuracy densitometer which is capable of operation under the high temperature, pressure, shock and vibration conditions encountered in a wellbore; which uses relatively inexpensive components; which substantially eliminates any error caused by changing characteristics of any of the electronic components; and which effectively eliminates the errors associated with the effects of temperature and pressure on the system.
Accordingly, there is disclosed herein a measurement device for determining fluid properties from vibration frequencies of a sample cavity and a reference cavity. In one embodiment, the measurement device includes a sample flow tube, a reference flow tube, vibration sources and detectors mounted on the tubes, and a measurement module. The sample flow tube receives a flow of sample fluid for characterization. The reference flow tube is filled with a reference fluid having well-characterized properties. The reference flow tube may be pressure balanced to the same pressure as the sample. The measurement module employs the vibration sources to generate vibrations in both tubes. The measurement module combines the signals from the vibration detectors on the tubes to determine properties of the sample fluid, such as density, viscosity, compressibility, water fraction, and bubble size. The measurement module may further detect certain flow patterns such as slug flow, for example.
To determine the sample fluid density, the measurement module measures the difference between resonance frequencies of the sample flow tube and the reference flow tube. The density can then be calculated according to a formula. Other fluid properties may be determined from the sample tube""s resonance peak amplitude, peak width and/or peak shape. Variation of the density measurements may be used to detect and characterize multiple phase fluid flow. The use of a reference tube in the disclosed measurement device is expected to greatly enhance the accuracy and reliability of the measurement device over a range of temperatures, pressures, and shock accelerations such as those that may be found in a borehole.